A conventional automatic blood pressure gauge includes a resilient inflatable cuff and an electric pump. The pump is controlled by a microprocessor to inflate the cuff with a fluid, such as air from the ambient environment, to a preset pressure. In addition, this automatic gauge includes a pressure transducer that measures the instantaneous air pressure levels in the cuff. The pressure signa) produced by the transducer is used to determine both the instantaneous air pressure of the cuff and the blood pressure pulse of the individual. This pressure signal is generally digitized and processed by the microprocessor to produce values representing the systolic and diastolic blood pressure measurements of the individual.
In operation, the cuff is affixed to the upper arm area of the patient and is then inflated to a pressure greater than the suspected systolic pressure, for example, 150 to 200 millimeters of mercury (mmHg). This pressure level collapses the main artery in the arm, effectively stopping any blood flow to the lower arm. Next, the cuff is deflated slowly and the transducer pressure signal is monitored to detect variations in cuff pressure caused by the patient's pulse, which is coupled into the cuff. By monitoring the amplitude of the measured pulse signal, the system can determine the patient's systolic and diastolic pressures.
One exemplary system is described in U.S. Pat. No. 4,949,710 entitled METHOD OF ARTIFACT REJECTION FOR NONINVASIVE BLOOD-PRESSURE MEASUREMENT BY PREDICTION AND ADJUSTMENT OF BLOOD-PRESSURE DATA, which is hereby incorporated by reference for its teaching on automatic blood pressure gauges. This system monitors the patient's blood pressure signal to determine the maximum detected pulse amplitude. This is commonly referred to as the mean arterial pressure (MAP). The systolic and diastolic blood pressure levels are then determined as the respective pressures corresponding to the amplitude of the pulse signal being 60% of the maximum value, prior to reaching the maximum value; and 80% of the maximum value, after reaching the maximum value.
To be most effective, an automatic blood pressure gauge should quickly inflate the cuff to a preset pressure value and then deflate the cuff according to a known deflation curve. It is desirable to complete this task in a relatively short time period, so as to provide quick results and to minimize patient anxiety and discomfort.
The pump, described above, permits quick inflation and one or more valves permit deflation. However, these pumps and valves are likely to cause pressure waves and acoustic noise. The pressure waves could propagate through the system, be detected by the pressure transducer and, so, interfere with the detection of the pulse data. In addition, the acoustic noise tends to be an annoyance for the user and the patient, especially if the measurement system is used over long periods of time or frequently used.
One exemplary system for decreasing the noise of a blood pressure monitoring system is described in U.S. Pat. No. 4,949,710 entitled LINEAR, LOW NOISE INFLATION SYSTEM FOR BLOOD PRESSURE MONITOR. This system uses two acoustic filters, one coupled to the inlet side of a pump for inflating a cuff and the other coupled to the outlet side of the pump. The pressure waves produced by the pump are propagated through longitudinal bores in the acoustic filters, then attenuated in resonance chambers coupled to the longitudinal bores by Way of transverse bores.